A canal of the Venice Lagoon laps across the sill of a historic doorway shuttered with iron. Built on a sinking marsh, Venice floods often due to tides and weather. On one island, a Roman walkway is now 5 feet below sea level. Industrial pumping of groundwater (now banned) sank Venice by 10 centimeters from 1920-1970. Global warming now raises sea levels by 3.2 centimeters (1.3 inches) per decade, much faster than the marsh sediments are compacting downwards. An overwhelming consensus of world scientists agree that global warming is indeed happening and humans are contributing to it through emission of heat-trapping "greenhouse gases," primarily carbon dioxide (see www.ucsusa.org). Since the industrial revolution began, humans have increased atmospheric CO2 concentration by 35% (through burning of fossil fuels, deforesting land, and grazing livestock). The Republic of Venice wielded major sea power during the Middle Ages, Crusades, and Renaissance. Riches from Venice's silk, grain, and spice trade in the 1200s-1600s built elaborate architecture combining Gothic, Byzantine, and Arab styles. Venice and the Venetian Lagoon are honored on UNESCO's World Heritage List.